Combustible gas, such as natural gas or hydrogen gas, is gas at a normal temperature. Therefore, when storing or transporting the gas, the gas is liquefied and held in the heat-insulating container. When the natural gas that is the combustible gas is used as an example, representative examples of the heat-insulating container which holds the liquefied natural gas (LNG) include an LNG storage tank installed on the ground, or a tank of an LNG transport tanker. The LNG is a low-temperature substance of which the temperature (the temperature of the LNG is normally −162° C.) is lower than the normal temperature by 100° C. or more. Therefore, in the LNG tanks, it is required to store the low-temperature substance of which the temperature is lower than the normal temperature on the inside thereof, and to improve heat-insulating performance as high as possible.
As one example of the heat-insulating material having higher heat-insulating performance, a vacuum heat-insulating material which uses a fiber-like core material made of an inorganic material is known. A general vacuum heat-insulating material has a configuration in which the core material is sealed in a tightly closed and decompressed state, on the inside of a bag-like outer cover material having gas barrier characteristics. Examples of fields in which the vacuum heat-insulating material is employed include electronic appliances including a home refrigerator, refrigerating facility for office, and a heat-insulating wall for a house.
For example, in PTL 1, a configuration in which the vacuum heat-insulating material is employed in a heat-insulating container, such as an LNG tank. Accordingly, entering of heat into the heat-insulating container is efficiently suppressed, and further improvement of volume efficiency is expected. In the LNG tank, when the entering of the heat can be suppressed, it is possible to efficiently reduce generation of boil-off gas (BOG), and to efficiently decrease a boil off rate (BOR) of the LNG.
Here, if by any change, when a case where the heat-insulating container is exposed to water of liquid is assumed, in the vacuum heat-insulating material used in the heat-insulating container, when the outer cover material is damaged (that is, when the vacuum heat-insulating material is ruptured), it becomes apparent that it is necessary to assume that the water of the liquid enters the inside thereof, by investigation of the inventors.
For example, the liquefied gas tanker sails on the sea, and generally, comes into contact with the sea water. Therefore, when the vacuum heat-insulating material is ruptured by any accident, reaction between the sea water and the inner member can be generated at a reaction speed and with a reaction amount which are equal to or greater than assumed. This point is also similar to that of a case where the ruptured vacuum heat-insulating material comes into contact with rain water not only in the gas tanker but also in a gas tanker installed on the ground or under the ground.
However, in the heat-insulating container of the related art, influence generated by a chemical reaction between the inner member and the moisture when the vacuum heat-insulating material is ruptured, is not considered.